1. Field of the Invention
The present invention relates to a sheet thickness detector for determining the thickness of a sheet, and to an image forming apparatus using the sheet thickness detector.
2. Discussion of the Related Art
Electrophotographic image forming apparatuses, in which a toner image formed on an image bearing member is transferred onto a receiving sheet and the toner image is then fixed thereto by a fixing device, are known. When plain paper having a normal thickness and thick paper are used as the receiving sheet for such image forming apparatuses, the image forming apparatuses typically perform controlling operations such that the transfer conditions and fixation conditions are changed depending on the thickness of the receiving sheet used. Specifically, such image forming apparatuses determine the thickness of the receiving sheet using a sheet thickness detector, and control the transfer conditions and fixation conditions depending on the thickness of the receiving sheet. Such a sheet thickness detector is required to provide high detection precision at low cost.
It is also preferable for image forming apparatuses other than the above-mentioned electrophotographic image forming apparatuses to determine the thickness of a receiving sheet to perform controlling operations depending on the thickness of the receiving sheet used. For example, some inkjet recording apparatuses determine the thickness of a receiving sheet to perform a controlling operation such that a predetermined distance, or gap, between the surface of the sheet and the surface of the inkjet recording head is maintained even when the thickness of the receiving sheet changes.
Thus, it is known to determine the thickness of a sheet used for image formation. Specific examples of such sheet thickness detectors include devices in which a lever (terminal) of a contact displacement sensor is directly contacted with a movable roller (hereinafter referred to as a displacement roller) to determine the amount of displacement of the roller caused by the sheet, and thus the thickness of the sheet. However, the accuracy of such contact-type sensors is known to deteriorate over time.
Non-contact types of sheet thickness detectors are also known that use optical, magnetic, or other technologies to detect sheet thickness. Specific examples of such non-contact types of sheet thickness detectors include a device measuring the amount of displacement of a displacement roller using a non-contact displacement sensor; a device determining the thickness of a sheet depending on the amount of light passing through the sheet; a device detecting a sheet having a thickness greater than a predetermined thickness depending on whether a flag provided on an end of a lever contacted with a displacement roller and swinging due to swinging of the displacement roller interrupts an optical sensor; and a device measuring the amount of displacement of a displacement roller using a non-contact magnetic displacement sensor.